Precure tread for a new tire, and retread tires

ABSTRACT

A precure tread for a tire has surface grooves facing a tread face, rear grooves facing a crown portion of a base tire, and a passage for communication between the rear grooves to atmosphere. The groove bottom of the rear grooves is located at a higher level than a groove bottom of the surface groove in the radial direction. The tire durability can be considerably improved when this precure tread is applied to a new tire or a retread tire.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a precure tread for a tire, pneumatic tiresusing the same and a method of producing such a pneumatic tire.Particularly, the invention provides a considerable improvement of treaddurability when this precure tread is applied to, for example, a new orretreaded tire for truck and bus use.

2. Related Art Statement

In tires, such as a pneumatic tire for truck and bus use and the like,as the running distance increases, a tread 2 of a tire 1 is graduallyworn as shown in FIG. 19 to shallow the depth of circumferential maingrooves 3 formed on the surface of the tread 2, whereby the roadgripping force of the tread 2 as well as the wet performances of thetire greatly lower. Therefore, the tire 1 is generally and frequentlyretreaded at a state that the main groove 3 is left to a certain extentin the tread 2.

The retreading of the tire is usually carried out by buffing theremaining tread portion 2a of the worn tire 1 to form a base tire 4 asshown, for example, by dotted lines in FIG. 19, and then bonding aprecure tread 6 to a crown portion 4a of the base tire 4 through acushion rubber 5 by vulcanization as shown in FIG. 20.

Moreover, the retreading of the tire is repeated 2 to 3 times or may berepeated more than 3 times though it changes in accordance with theservice conditions. In case of repeating the retreading, if the wearresistance of the tread is low or the wet performances and the like atlast worn stage are low, the period between the retreading and nextretreading becomes shorter and the retreading becomes uneconomical.

In order to sufficiently develop the wet performances and otherperformances of the tread even at the last worn stage, there is proposeda tire as described in Japanese Utility Model laid open No. 51-95,902.

In this proposed tire, a rear groove extending from a position separatedoutward from the bottom of the surface groove in the radial directiontoward the inside in the radial direction is continuously ordiscontinuously disposed in the inside of the tread in thecircumferential direction of the tire. In this tire, the rear groove isexposed at the last worn stage, whereby the degradation of the wetperformances and other performances can effectively be prevented. Thatis, when this proposal is applied to a precure tread 8 of a retreadedtire 7 as shown in FIG. 21, the precure tread 8 comprises plural maingrooves 3 at its ground contacting portion 8a and a rear groove 9 at itsrear the tread 8b facing the base tire. When the tread 8 is worn up to aposition shown by dot-dash lines M in FIG. 21 during the use of theretreaded tire 7, the rear groove 9 appears at a tread face 8c, andconsequently the drainage effect through the rear groove 9 as well asthe edge effect at the groove edge of the rear groove are generated tolargely improve the wet performances.

Furthermore, the precure tread 8 is designed to provide sufficient wetperformances at the last worn stage, so that it is useless to ensureextra wet performances in the precured tread over a period ranging frominitial worn stage to last worn stage. As a result, a ratio of surfacegroove area occupied in ground contacting area of the tread or so-callednegative ratio can be made small to improve the wear resistance andresistance to uneven wear.

In the tire of FIG. 21, however, the rear groove 9 forms a closed spacethrough the bonding of the precure tread 8 to the cushion rubber 10, sothat gas existing in the rear groove 9 thermally expands throughcompressive deformation of the tread 8 repeated during the running ofthe tire and gets into an interface between the tread 8 and the cushionrubber 10 due to the increase of the volume of the gas itself. As aresult, there is a problem of peeling a portion of the tread 8 near therear groove 9 from the cushion rubber 10 as exaggeratedly shown in FIG.21 to degrade the durability of the tire.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to advantageously solve theabove problems of the conventional technique and to provide a precuretread for a tire capable of considerably improving the durability of thetread and hence the tire by sufficiently preventing the peeling of thetread due to the presence of the rear groove, a pneumatic tire using thesame, and a method of producing such a pneumatic tire.

According to a first aspect of the invention, there is the provision ofa precure tread for a tire to be bonded to a crown portion of a basetire and having a ground contacting portion located at a tread side ofthe tire and a rear portion located at the side of the base tire inwhich plural surface grooves are formed in the ground contacting portionand plural rear grooves are formed in the rear portion. A groove bottomof the rear groove is located at a level higher than a groove bottom ofthe surface groove in the radial direction of the tire. Each of the reargrooves is communicated to an atmosphere at the side of said groundcontacting portion through a passage formed in said precured tread.

According to a second aspect of the invention, there is the provision ofa pneumatic tire obtained by integrally bonding a base tire to a precuretread for a tire having a ground contacting portion located at a treadside of the tire and a rear portion located at the side of the basetire. Plural surface grooves are formed in the ground contacting portionand plural rear grooves are formed in the rear portion, through anadhesive or an unvulcanized cushion rubber. A groove bottom of the reargroove is located at a level higher than a groove bottom of the surfacegroove in the radial direction of the tire. Each of the rear grooves iscommunicates to atmosphere at the side of said ground contacting portionthrough a passage formed in said precure tread.

The term "precure tread" used herein means a tread formed in a mold andvulcanized therein. The rear groove may be continuously ordiscontinuously formed in the circumferential direction of the tire,while the sectional shape of the passage may be circle of venthole-like,rectangle of sipe-like, and other required shape.

According to a third aspect of the invention, there is the provision ofa method of producing a pneumatic tire as defined in the secondinvention, comprising attaching a precure tread having a groundcontacting portion located at a tread side of the tire and a rearportion located at a side of a base tire. Plural surface grooves areformed in the ground contacting portion and plural rear grooves areformed in the rear portion, to a crown portion of a base tire through anunvulcanized cushion rubber, mounting the resulting assembly onto a rimtogether with an envelop housing the assembly therein, and thenvulcanizing the assembly by heating under a pressure. The rear groovesare communicate to atmosphere through a passage formed in the precuretread. A wick contacts with the outer peripheral surface of the precuretread and a guide pipe contacts with the wick.

In a precure tread according to a fourth aspect of the invention, thegroove bottom of the rear groove is located at a level higher than thegroove bottom of the surface groove, and an opening portion extending tothe surface of the tread is formed in the precure tread above the reargroove and separated from the rear groove through a partition wall. In aprecure tread according to a fifth aspect of the invention, the groovebottom of the rear groove is located at a level higher than the groovebottom of the surface groove, and an opening portion of the rear grooveextending above the rear groove is formed in the precure tread andterminated in a protrusion formed on the surface of the tread surface.

According to a sixth aspect of the invention, there is the provision ofa method of producing a pneumatic tire, comprising the steps of:

(a) attaching a precure tread having a ground contacting portion locatedat a tread side of the tire and a rear portion located at a side of abase tire, in which plural surface grooves are formed in the groundcontacting portion and plural rear grooves are formed in the rearportion, to a crown portion of a base tire through an unvulcanizedcushion rubber;

(b) mounting the resulting assembly onto a rim together with an envelophousing the assembly therein;

(c) vulcanizing the assembly by heating under a pressure; and

(d) communicating said rear groove to an atmosphere after thevulcanization.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein:

FIGS. 1 to 3 show a first embodiment of the precure tread according tothe first aspect of this invention and the pneumatic tire using thesame, wherein FIG. 1 is a partly plan view of the tire, FIG. 2 is asectional view taken along a line II--II of FIG. 1, and FIG. 3 is apartly sectional view in widthwise direction of the base tire used inthe manufacture of the retreaded tire of FIG. 1;

FIGS. 4 and 5 are partly sectional views showing a main part of theprecure tread for tire according to the invention, respectively;

FIGS. 6 to 13 show second to fifth embodiments of the precure treadaccording to the invention, wherein FIG. 6 is a partly plan view of thesecond embodiment, FIG. 7 is a sectional view taken along a lineVII--VII of FIG. 6, FIG. 8 is a partly plan view of the thirdembodiment, FIG. 9 is a sectional view taken along a line IX--IX of FIG.8, FIG. 10 is partly plan view of the fourth embodiment, FIG. 11 is apartly plan view of the fifth embodiment, and FIGS. 12 and 13 aresectional views taken along lines XII--XII and XIII--XIII of FIG. 11,respectively;

FIG. 14 is a sectional view illustrating a method for the production oftire using the precure tread for tire according to the first aspect ofthis invention;

FIGS. 15 and 16 are sectional views in widthwise direction of the sixthand seventh precure treads, respectively;

FIGS. 17a-e and 18a-b are diagrammatical views illustrating variousmodifications in the plan and section forms of the protrusion,respectively;

FIG. 19 is a sectional view in widthwise direction of a tire to beretreaded;

FIG. 20 is a sectional view in widthwise direction of the conventionalretreaded tire; and

FIG. 21 is a sectional view in widthwise direction of anotherconventional tire.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

When the first precure tread according to the invention is applied to anew tire or a retreaded tire, the rear groove is exposed before thedisappearance of the surface groove with the advance of tread wear. Thatis, the groove as expected is existent in the tread over a periodranging from the initial worn stage to the last worn stage, so thatthere is no fear of lowering the drainage and wet performances of thetire even when the tread is worn up to the rear groove. Consequently thewear life of the tread is considerably prolonged.

In this case, the surface groove can be arranged in the precure treadwithout particularly considering the wet performances of the tread atthe last worn stage. That is, it is sufficient enough to consider onlythe wet performances ranging from the initial worn stage of the tread tothe last worn stage thereof, so that the wear resistance of the treadcan largely be improved by effectively reducing the ratio of surfacegrooves occupied in the tread surface.

In case of tires using the above precure tread, since the rear groovecommunicates to atmosphere through the passage formed in the precuretread, gas inside the rear groove is freely moved toward the atmosphere,so that even when heat is generated in the tread due to compressiondeformation of the tread during the running, there is no peeling of thetread resulted from thermal expansion of gas inside the rear groove.Further, the tread is effectively cooled by air flowing in or out in therear groove, so that the thermal fatigue of the tread and cushion rubbercan sufficiently be controlled.

In the first method according to the invention, when the precure treadis bonded by vulcanization to the crown portion of the base tire throughthe unvulcanized cushion rubber, the rear groove of the precure treadcommunicates to atmosphere through the passage formed in the precuretread, the wick contacting with the outer peripheral surface of theprecure tread and the guide pipe contacting with the wick. Thus, airsandwiched between the precure tread and the unvulcanized cushionrubber, gas produced by the reaction of the unvulcanized cushion rubberand the like are smoothly discharged into the atmosphere, so that theadhesion force between the cushion rubber and the tread is considerablyincreased to largely improve the durability of the tire.

In the second and third precure treads according to the invention, thecommunication of the rear groove to the atmosphere is obstructed by thepartition wall or the protrusion formed on the tread surface. In thiscase, however, the rear groove can communicates very simply toatmosphere at the side of the ground contacting portion of the precuretread by piercing a hole in the partition wall with the opening portionfacing the tread surface as a mark or cutting out the protrusion fromthe precure tread at as-precured tread state or after the bonding to thebase tire.

Moreover, the partition wall or the protrusion in such a precure treadcan be removed in accordance with the wearing of the tread when theprecure tread is bonded to the base tire and then mounted onto a vehicleand run over a given distance.

In the second method for the manufacture of the tire according to theinvention, the vulcanization is carried out by using the above secondand third precure treads and then the rear groove is communicates toatmosphere by the piercing of the partition wall or the removal of theprotrusion after the vulcanization. Thus the peeling of tread resultingfrom thermal expansion of gas in the rear groove can be prevented veryeffectively.

In case of applying the second precure tread, the given piercingposition can be simply and rapidly found owing to the presence of theopening portion facing the tread surface. In case of applying the thirdprecure tread, all protrusions are cut out by means of a cutter or thelike, so that the rear groove can be communicated to atmosphere verysimply and rapidly at small operating steps as mentioned above.

FIGS. 1 to 3 illustrate the first precure tread according to theinvention and a first embodiment of the pneumatic tire using the same.

The construction of such a tire will be described below.

In FIGS. 1 and 2, numeral 11 is a retreaded tire for truck and bus, andnumeral 12 a precure tread applied for the retreaded tire 11. Thisprecure tread 12 is composed of a vulcanized rubber member andintegrally united with a base tire 13 by bonding to a crown portion 13aof the base tire 13, which is obtained by buffing a tread 14 of a usedtire shown by a phantom line in FIG. 3 to solid line position, through asheet-like cushion rubber 15 by vulcanization.

In FIGS. 2 and 3, numeral 14B is a belt.

As shown in FIGS. 1 and 2, the precure tread 12 is provided at a groundcontacting portion 12b facing a side of a tread face 12a with three maingrooves 16 as a surface groove extending zigzag in the circumferentialdirection of the tire. These main grooves 16 are designed to improve thewear resistance by reducing the groove area in view of the considerationof only the wet performance over a period from the initial worn stage tothe last worn stage.

Numeral 17 is a sipe, which is obtained by arranging a short sipe in ashoulder portion 12c of the precure tread 12 and both side edge portionsof the main groove 16 in the widthwise direction of the tire.

Numeral 18 is a rear groove. In the precure tread 12, four rear grooves18 are formed in a rear portion 12d of the precure tread 12 facing thebase tire 13 between the main grooves 16 and between the main groove 16and the shoulder portion 12c. Each of the rear grooves 18 is directedfrom the side of the base tire 13 to the side of the tread face 12a andcontinuously extends zigzag in the circumferential direction of thetire.

Moreover, the main groove 16 is substantially U-shaped in section andhas preferably a relation of D₁₂ =D₁₆ +(3˜4 mm), wherein D₁₆ is a depthof the main groove 16 and D₁₂ is a thickness of the precure tread 12. Onthe other hand, the rear groove is also substantially U-shaped insection and has a relation of D₁₈ =D₁₆ ×0.4+(3˜4 mm), wherein D₁₈ is adepth of the rear groove 18. That is, the main groove 16 and the reargroove 18 are exposed to atmosphere together when the precure tread 12is worn to a given extent.

When the retreaded tire 11 using such a precure tread 12 is mounted ontoa truck or bus and actually run, as the running distance increases, theprecure tread 12 of the retreaded tire 11 is worn from the side of thetread face 12a to gradually make the depth of the main groove 16shallow. When the worn quantity of the precure tread 12 reaches about60% of the thickness D₁₆, the rear grooves 18 appear. In other words,the rear groove is exposed before the complete disappearance of the maingroove 16, so that both the main groove 16 and the rear groove 18contribute to the improvement of the wet performances. Therefore, theretreaded tire 11 using this precure tread 12 can sufficiently developthe drainage and wet performances on wet road surface even at the lastworn stage of the tread. Furthermore, the retreaded tire can be useduntil the rear groove disappears, so that the wear life of the tire canlargely be prolonged.

For this end, the depth D₁₈ of the rear groove 18 should exceed over aposition corresponding to the groove bottom 16a (D₁₆) of the main groove16 as shown in FIG. 4. Preferably, the rear groove 18 has a depthD_(18A) corresponding to a point 16b 0.4 times of the depth D₁₆ of themain groove 16. Further, the groove width W₁₈ of the rear groove 18should preferably be not less than 1 mm.

According to the invention, the sectional form of the rear groove 18 isnot restricted to only the U-shaped form. For example, as shown in FIG.5, a rear groove 18A has a V-shaped form in section in which the groovewall has an angle a with respect to a line segment N perpendicular tothe rear face 12e of the precure tread 12. A rear groove 18B having ateardrop like form in section in which the groove wall is inclined at anangle β with respect to the line segment N in an opposite direction tothe case of the rear groove 18A, and the like.

In FIGS. 1 and 2, numerals 20 and 21 are a venthole and siperespectively formed in the precure tread 12 as a passage forcommunicating the rear groove to the atmosphere, respectively. Theventhole 20 is arranged in each folded portion 18a of the rear groove 18and preferably has a diameter of not less than 0.5 mm. Moreover, thenumber of ventholes is preferably not less than 5 on the circumferencewhen the rear groove 18 continuously extends in the circumferentialdirection. When the rear groove 18 is discontinuously extended in thecircumferential direction, 5 or more ventholes are arranged in each ofthe discontinued groove portions.

The sipe 21 arranged in the straight portion 18b of the rear groove 18preferably has an opening width of not less than 0.5 mm and a length ofnot less than 3.0 mm along the rear groove 18.

Further, the number of sipes is preferably not less than 5 on thecircumference when the rear groove 18 continuously extends in thecircumferential direction. When the rear groove 18 is discontinuouslyextended in the circumferential direction, 5 or more sipes are arrangedin each of the discontinued groove portions.

Since the rear groove 18 always communicates to atmosphere at the sideof the ground contacting portion through these ventholes and sipes, whenthe retreaded tire 11 using the precure tread 12 is mounted onto a truckor bus and actually run, even if the gas inside the rear groove 18 isthermally expanded by heat generated from rubber around the rear groove18 due to the compression deformation of the tread during running, suchgas is discharged toward the outside through the venthole 20 and/or thesipe 21. Thus the penetration of the expanded gas between the tread 12and the cushion rubber 15 and hence the peeling of the tread 12 from thecushion rubber 15 are prevented very effectively. At the same time,cooled air flows from the outside into the rear groove 18 through theventhole and/or sipe to cool the groove walls of the rear groove 18 andthe cushion rubber 15, so that the thermal fatigue of the tread 12 andthe cushion rubber 15 is sufficiently restrained.

The second embodiment of the invention will be described below.

FIGS. 6 and 7 show the second embodiment of the precure tread accordingto the invention, wherein the same element as in the first embodiment isrepresented by the same numeral. In this precure tread 26, two maingrooves 16 are arranged in the tread face 12a to continuously extendzigzag in the circumferential direction of the tread, and two reargrooves 18 are arranged between main grooves 16 and in the central partof the rear portion 12d to continuously extend in a zigzag in thecircumferential direction. Further, only the sipe 21 is arranged in theprecure tread 26 over a region ranging from a middle position of thestraight portion 18b of the rear groove 18 toward the folded portion 18athereof.

The third to fifth embodiments of the invention will be described below.The same parts as in the first embodiment are represented by the samenumeral.

FIGS. 8 and 9 show the third embodiment of the precure tread accordingto the invention.

In this precure tread 31, many folded rear grooves 28 discontinuouslyextending in the circumferential direction are arranged in the rearportion 12d of the precure tread between the tread shoulder portion 12cand the main groove 16 adjoining thereto. The venthole 20 is arranged inthe precure tread at each folded portion of the rear grooves 28.

FIG. 10 shows a fourth embodiment of the precure tread according to theinvention.

In this precure tread 41, a rear groove 18 continuously extending in azigzag in the circumferential direction is arranged in the rear portion12d located at the central portion of the precure tread. The venthole 20is arranged in the precure tread at the folded portion 18a of the reargroove 18.

FIGS. 11 to 13 show a fifth embodiment of the precure tread according tothe invention.

In this precure tread 51, a rear groove 18 continuously extending in azigzag in the circumferential direction is arranged in the rear portion12d located at the central portion of the precure tread. A wide sipe 21is arranged in the precure tread at the straight portion 18b of the reargroove 18.

In order to confirm the effect of the aforementioned precure treadsaccording to the invention, there were provided three test tires (tireI˜II) and then the durability test was made with respect to these tires.The durability test will be described below.

The test tire I is the first embodiment, wherein only 4 ventholes eachhaving a diameter of 2 mm are arranged on the circumference forcommunicating each of the rear grooves to the atmosphere. The test tireII is a modified example of the test tire I, wherein 12 ventholes arearranged on the circumference. Further, the test tire III is also amodified example of the test I, wherein only 5 ventholes are arranged onthe circumference.

These test tires were subjected to a durability test by means of anin-door type drum testing machine under usual durability testconditions. As a result, the test tire I was broken at the runningdistance of 3304 km, and the test tire II was broken at the runningdistance of 5292 km, and the test tire III was broken at the runningdistance of 5180 km.

As seen from the above, the test tires II and III have substantially thesame level in the durability, while in the test tire I, the effect ofcommunication to atmosphere is insufficient.

Further, when each of the test tires II and III is mounted onto avehicle and actually run, the vehicle can normally be run even when theworn quantity of the tread is 75% ˜92%, whereby the wear life of thetire can largely be improved.

On the other hand, it have been confirmed that durability is furthermore improved by enhancing the fatigue resistance of the cushion rubber.

As mentioned above, according to the invention, the wet performances onwet road surface can sufficiently be developed even at the last wornstage of the tread and also the wear life can largely be improvedwithout damaging the wet performances.

Moreover, the groove area of the surface grooves can be reduced over aperiod reaching to the last worn stage, whereby the substantial groundcontacting area of the tread can be increased to realize the improvementof the wear resistance of the tread.

FIG. 14 is a sectional view illustrating a first method for themanufacture of tires according to the invention using the above firstembodiment of the precure tread for tire. According to this method, airsandwiched between the precure tread 12 and unvulcanized cushion rubberwhen the precure tread 12 is attached to the outer peripheral surface ofthe unvulcanized cushion rubber, and reaction gas produced from thecushion rubber when the precure tread 12 is bonded to the base tire 13by vulcanization are sufficiently discharged from the rear grooves 18.Thus the adhesion force of the precure tread 12 to the cushion rubber 15and hence the tire durability can be improved.

In the first method, the precure tread 12 is first attached to the crownportion 13a of the base tire 13 through the unvulcanized cushion rubber15a to form an attached assembly 61. Then, a wick 62 extending in thewidthwise direction of the assembly 61 and arriving inward in the radialdirection over the arranged position of the unvulcanized cushion rubber15a is attached to at least one portion of the assembly 61 on the outerperiphery thereof, while a protection liner (not shown) is attached tothe remaining portion of the outer periphery of the assembly 61 otherthan the position of the wick 62. Next, the assembly 61, the wick 62 andthe protection liner are enveloped with an envelop 63 to contact a topof a guide pipe 64 attached to the envelop 63 with the outer peripheryof the wick 62. Thereafter, the assembly 61 is lined with an inner tube65 and mounted into a rim 66 together with the envelop 63. Then,pressurized air of, for example, about 8 kg/cm² is supplied to theinside of the inner tube 65 through a valve 67.

In this case, the attachment of the inner end portion of the envelop 63in the radial direction to the rim can be carried out by sandwichinganother flange member 68 between the envelop and the rim flange toairtightly seal the envelop 63 and the assembly 61, whereby theintroduction of air or other gas therebetween can sufficiently beprevented.

After the resulting rimmed structure 69 is placed in a vulcanizer 70,the other end portion of the guide pipe 64 is led from the vulcanizer 70to the outside thereof and then the vulcanizer 70 is closed. Thereafter,the assembly 61 is vulcanized by supplying a heated pressurized fluid tothe inside of the vulcanizer 70. Such vulcanization can be carried outat a temperature inside the vulcanizer of about 130° C., for example, bysupplying steam to the inside of the vulcanizer under a pressure ofabout 2 kg/cm² and further feeding air until the inner pressure of thevulcanizer reaches about 6 kg/cm².

When the precure tread 12 is bonded by vulcanization to the base tire 13as mentioned above, air sandwiched between the assembly 61 and theenvelop 63 moves toward the wick 62 under an influence of the pressureapplied to the envelop 63. It then is discharged to the atmosphereoutside the vulcanizer 70 through the guide pipe 64 contacting at itstop with the wick 62, while air sandwiched between the unvulcanizedcushion rubber 15a and the precure tread 12, reaction gas produced fromthe cushion rubber and the like mainly arrive at the wick 62 arranged onat least one portion of the assembly 61 in the circumferential directionand contacted with the tread 12 through the rear grooves 18, ventholes20 and/or sipes 21 and then discharged to the atmosphere through theguide pipe 64.

In this way, the inner pressure of the rear groove is always maintainedat a pressure equal to an atmospheric pressure. Therefore, the pooradhesion between the precure tread 12 and the cushion rubber 15 due tothe fact that air, gas and the like remain in the rear grooves under apressure higher than the atmospheric pressure is completely eliminatedand the sufficiently strong adhesion can be realized.

When the precure tread 12 is bonded by vulcanization as mentioned above,it is preferable that in order to prevent the introduction of thecushion rubber into the rear groove during the vulcanization, the groovewidth of the rear groove 18 is 2˜5 mm, and the inner pressure of thevulcanizer is within a range of 2˜6 kg/cm², and the cushion rubber has aMooney viscosity of not less than 40 when it is preheated for 1 minuteand cured at 130° C. for 4 minutes. Thus, there can advantageously beprevented the lowering of the drainage performance at the last wornstage due to the fact that the lower end portion of the rear groove oran inward portion of the rear groove in the radial direction of the tireis buried with the cushion rubber 15, and the occurrence of cracking inthe cushion rubber due to the fact that the cushion rubber complicatedlyflows.

In the manufacture of tires according to the first method, there may bea fear that the main groove 16 is thermally deformed in connection withthe size and form of the main groove 16. In this case, after a giventime since the beginning of the vulcanization, air is supplied into themain groove through the guide pipe 64 under a pressure lower than theinner pressure of the vulcanizer, for example, a pressure of about 5kg/cm², whereby the thermal deformation of the main groove 16 cansufficiently be removed without exerting on the adhesion force betweenthe precure tread 12 and the cushion rubber 15.

The comparison test for durability and tread adhesion force will now bedescribed with respect to a tire manufactured according to the firstmethod of the invention and a tire manufactured without communicatingthe rear groove to the atmosphere.

Test tires:

a) Invention tire

Each of the rear grooves is opened to the tread face of the tire through12 ventholes having a diameter of 2 mm.

b) Comparative tire

All ventholes are omitted from the invention tire.

Test method:

As to the tire durability, the drum test was carried out with respect tothe test tire to measure the running distance until the occurrence ofthe peeling between belt layers or the peeling at the retreadedinterface. As to tread adhesion force, the peeling resistant forcebetween tread and cushion rubber per a width of 1 inch was measured bymeans of a tensile testing machine.

Test results:

The results are shown in the following Table 1 by an index on the basisthat the comparative tire is 100. The larger the index value, the betterthe property.

                  TABLE 1                                                         ______________________________________                                                     Comparative tire                                                                         Invention tire                                        ______________________________________                                        Tire durability                                                                              100          300                                               Tread adhesion force                                                                         100          120                                               ______________________________________                                    

As seen from the above results, both tire durability and tread adhesionforce can largely be improved according to the first method of theinvention.

According to the first method of the invention, when the precure tread12 is bonded by vulcanization to the base tire 13 through the cushionrubber, the rear groove communicates to atmosphere through the passagearranged in the precure tread 12, the wick 62 contacting with theprecure tread 12 and the guide pipe 64 contacting with the wick 62 todischarge air, reaction gas and the like to the outside, whereby thetire durability can largely be improved while increasing the adhesionforce of the tread 12.

Moreover, the rear grooves 18 are indirectly connected to the guide pipe64 through the wick 62, so that undesirable gases exerting on theadhesion performance can completely and concentrically dischargedthrough the single guide pipe 64.

FIG. 15 is a sectional view in widthwise direction of a sixth embodimentof the precure tread according to the invention.

In this precure tread 71, the main grooves 16 and the rear grooves 18are arranged likewise the precure tread 12. At least one opening portion72 opened to a tread face 71a is arranged above each of the rear grooves18 and separated from the rear groove 18 through a partition wall 73. Inthis case, the opening portion 72 has a diameter a of 0.5˜4.0 mm and adepth h of 0.5˜8.0 mm, and the partition wall 73 has a thickness b of0.5˜6.0 mm. These values of diameter, depth and thickness respectivelycan properly be selected within the above ranges, if necessary.

In the precure tread 71, the reason why the diameter a of the openingportion 72 is within a range of 0.5˜4.0 mm, preferably 2.0˜3.0 mm isbased on the fact that the flowing of gas is sufficiently smoothened andthe required rigidity of the island portions in the tread is ensured.The reason why the depth h of the opening portion 72 is within a rangeof 0.5˜8.0 mm, preferably 1.0˜2.0 mm is based on the fact that thedurability of the mold pin is ensured and the deviation from center inthe communication between the opening portion 72 and the rear groove ismade small. Further, the reason why the thickness b of the partitionwall 73 is within a range of 0.5˜6.0 mm, preferably 2.0˜3.0 mm is basedon the fact that the sufficient pressure resistance of the partitionwall and the easiness of the subsequent piercing operation aresimultaneously established.

After the precure tread 71 is bonded by vulcanization to the base tire13, the whole or a part of the partition wall 73 is removed by a drillor other piercing means with the opening portion 72 as a mark toproperly communicate the rear groove 18 to the opening portion 72,whereby the desired tire is obtained.

Even when heat is generated in the tread during the running of the tire,gases inside the rear groove can be freely be discharged to the outsideof the tire likewise the aforementioned tire 11, so that the peeling oftread due to the thermal expansion of gas inside the rear groove can beeliminated very effectively to largely improve the tire durability.

Moreover, the removal of the whole or a part of the partition wall 73can be carried out by the piercing operation before the adhesion of theprecure tread 71 to the base tire 13, or by the wearing of tire surfaceduring the running under load after the adhesion to the base tire 13.

FIG. 16 is a sectional view in widthwise direction of a seventhembodiment of the precure tread according to the invention. In thisprecure tread 81, at least one rear opening portion 82 is arranged tocommunicate with the upper end of the rear groove 18, while the end ofthe rear opening portion 82 is terminated in a protrusion 83 disposed onthe tread face 81a.

In the precure tread 81, the diameter d of the rear opening portion 82can be selected within a range of 0.5˜4.0 mm, preferably 2.0˜3.0 mm inconnection with the flowability of gas and the rigidity required in theisland portions of the tread. Further, the penetration depth e of therear opening portion 82 into the protrusion 83 is about 0.5 mm when theprotrusion 83 can be cut out in the same plane as the tread face inconnection with the subsequent cutting accuracy of the protrusion, andis preferably about 1.0 mm. Moreover, the protruding height f of theprotrusion 83 can be selected within a range of 3.0˜10.0 mm inconnection with the cutting easiness and the yield of material, whilethe thickness g of sidewall of the protrusion 83 can be selected withina range of 0.5˜2.0 mm in connection with the pressure resistance and theyield of material.

The planform of the protrusion 83 can properly be selected from formsshown in FIGS. 17a to 17e and other forms, if necessary. On the otherhand, the sectional form of the protrusion may take a base portionnecked form as shown in FIGS. 18a and 18b for facilitating thesubsequent cutting operation.

In case of using the precure tread 81 for the manufacture of tires, theprecure tread 81 is first attached to the crown portion 13a of the basetire 13 through the unvulcanized cushion rubber 15a likewise the case ofusing the precure tread 71. The attached assembly is mounted onto a rim66 together with the envelop 63 and vulcanized in a vulcanizer byheating from the outside of the envelop 63 under a pressure. Aftervulcanization, the protrusion 83 protruding from the tread face 81a ofthe vulcanized assembly is removed by cutting out with a hand cutter orthe like.

According to the third method, many protrusions 83 can efficiently beremoved by means of the cutter, so that the number of steps required forcommunicating the rear groove 18 to the atmosphere can considerably bereduced as compared with the aforementioned case of piercing athrough-hole in the partition wall 73.

When the time required for cutting out 100 protrusions 83 is comparedwith the time required for forming the through-holes in 100 partitionwalls 73, the former case is 5 minutes, while the latter case is 30minutes.

Even in the resulting tire, the rear groove 18 is communicates toatmosphere during the use, so that the tire durability can largely beimproved likewise the use of the precure tread 71.

The removal of the protrusions 83 in the precure tread 81 can be carriedout before the adhesion to the base tire 13, or by the wearing duringthe running of the tire under a load.

Although the invention has been described with reference to theillustrated embodiments, the partition wall 73 in the precure tread 71may be located the same level as the tread face or a level near thereto.Also the lateral section of the opening portion 72 and the rear openingportion 83 may take sipe-like or other proper form, if necessary.

As mentioned above, according to the invention, the rear groovecommunicates to atmosphere at least in the use of the tire, so that thepeeling of tread due to the thermal expansion of gas inside the reargroove can sufficiently be prevented to largely improve tire durability.

In the first precure tread and the tire using the same, the wetperformances can sufficiently be developed even at the last worn stageof tread, and also the wear life can considerably be prolonged. Further,the wear resistance of tread can advantageously be improved by reducingthe negative ratio of the surface groove.

In the first method for the manufacture of tires according to theinvention, gas inside the rear groove can freely be discharged to theatmosphere, so that the adhesion force between the precure tread and thecushion rubber can be increased and the tire durability can largely beimproved.

In the second and third precure treads and the second method for themanufacture of tires using the same, the peeling of tread due to thethermal expansion of gas inside the rear groove can effectively beprevented, and also the rear groove can simply and easily communicate toatmosphere.

What is claimed is:
 1. A tire having a precure tread integrally bondedto a crown portion of a base tire, said precure tread comprising; aprecure tread ground contacting portion located at a tread surface sideof the tire and a rear portion located at the crown portion side of thebase tire, plural surface grooves formed in the ground contactingportion and open to said tread surface side, plural rear grooves formedin the rear portion and open to said base tire, a groove bottom of saidrear grooves being located at a level higher than groove bottoms of saidplural surface grooves in the radial direction of the base tire so thatsaid rear grooves are exposed prior to disappearance by wear of saidplural surface grooves, and a passage formed in said precure treadestablishing communication between said rear grooves and atmosphere atsaid ground contacting portion, said passage having a sectional sizesmaller than a sectional size of said surface groove.
 2. A precure treadas in claim 1 further comprising a sipe in said tread, said sipe havinga depth through said precure tread to said rear groove.
 3. A precuretread as in claim 1 wherein said rear groove is a zigzag grooveextending circumferentially.
 4. A precure tread as in claim 1 whereinsaid rear groove has a tapered sectional shape.
 5. A precure tread as inclaim 1 said passage further comprising a vent hole establishingcommunication to atmosphere.
 6. A pneumatic tire comprising; a precuretread integrally bonded to a base tire by adhesive or an unvulcanizedcushion rubber, said precure tread comprising; a ground contactingportion located at a tread surface side of the base tire and a rearportion located at a crown portion side of the base tire, plural surfacegrooves formed in the ground contacting portion and open to said treadsurface side, plural rear grooves formed in the rear portion, a groovebottom of said rear grooves being located at a level higher than groovebottoms of said plural surface grooves in the radial direction of thebase tire so that said rear grooves are exposed prior to disappearanceand wear of said plural grooves, and a passage formed in said precuredtread, establishing communication between said rear grooves andatmosphere at said ground contacting portion, said passage having asectional size smaller than a sectional size of said surface groove. 7.A pneumatic tire according to claim 6 further comprising a sipe in saidtread, said sipe having a depth through said precure tread to said reargroove.
 8. A pneumatic tire according to claim 6 wherein said reargroove is a zigzag groove extending circumferentially.
 9. A pneumatictire according to claim 6 wherein said rear groove has a taperedsectional shape.
 10. A pneumatic tire according to claim 6 said passagefurther comprising a vent hole establishing communication to atmosphere.